.nav{
    width:100%;
    justify-content: center;
    display: flex;
    flex-wrap: wrap;
}
.but{
    width: 200px;
    margin: 20px;
    height:50px;
    text-align: center;
}
.but a{
    width: 200px;
    height: 50px;
    line-height: 40px;
    font-size: 18px;
}